prompb: Update exposition protobuf to include float and gauge histograms

[beorn7]

This is in preparation of enabling ingestion of float and gauge histograms. The actual changes have happened in prometheus/client_model#58 , but since we use proto3 and gogoprotobuf in Prometheus, the changes have to be pulled in manually here.

@marctc

[codesome]

If I read it correctly, you added the float counts at 4 places, some additional comments, and some changes in the proto formatting.

[beorn7]

No, that's the actual trick here: There is no field gauge_histogram. There is only one field histogram in Metric. Whether it's a counter histogram or a gauge histogram is solely determined by this enum.

[beorn7]

Yeah, the re-formatting creates a lot of diff noise.

This should create/decode the same proto messages on the wire as prometheus/client_model#58 .

@marctc I'll merge this now. You can start working on ingestion, as discussed. Just no-op storing the FloatHistogram for now (with a TODO) and add an enum to Histogram and FloatHistogram to store the "counter-reset"/"no-counter-reset"/"don't know"/"gauge histogram" information. Let's call it ResetHint (as in the remote-write protobuf). I think there should be no active counter reset detection during scraping. It will only be "don't know" or "gauge histogram". (During actually storing the histogram, a counter reset is detected for the don't know case, which is already implemented. Just needs change for the new "gauge histogram" case.)